Third Harmonic Up‐Conversion in Elliptical Dimer‐Nanodisk Coupler Metasurfaces Supporting Quasi‐BIC Resonances

Abstract Resonantly enhanced frequency up‐conversion from 2380 to 793 nm wavelength is experimentally demonstrated through third harmonic generation (THG) leveraging quasi bound‐state‐in‐continuum (quasi‐BIC) resonances in a periodic array of amorphous silicon (a‐Si) elliptical dimers coupled to nanodisk couplers. The introduction of a subtle asymmetry in the form of a vertically displaced nanodisk coupler to an otherwise symmetric array allows access to BIC resonance. These resonances can be attributed to the electric quadrupole and magnetic dipole modes excited within the elliptical dimer through the electric dipole modes supported by the nanodisk coupler. Two such metasurfaces are fabricated by placing the nanodisk coupler either at the reflection symmetry point or a vertical offset of 500 nm to realize the asymmetric structure. Pronounced quasi‐BIC resonance is experimentally observed for the vertical offset nanodisk structure in the 2400 nm wavelength range with a quality factor of 151. Further, a resonant enhancement is observed in THG with a maximum enhancement of 312‐times when compared to the unpatterned a‐Si film, with a normalized THG conversion efficiency of 1.6 × 10 −2 W −2 and strong polarization dependence, consistent with the introduced asymmetry.